Variable resonant frequency spring-mass system device



Jan. 13, 1970 D. L. REXFORD VARIABLE RESONANT FREQUENCY SPRING-MASSSYSTEM DEVICE Filed July 30, 1968 W. fi

2 Sheets-Sheet 1 [/7 Ventar Jan. 13, 1970 REXFORD 3,489,15

VARIABLE RESONANT FREQUENCY SPRING-MASS SYSTEM DEVICE 2 Sheets-Sheet 2Filed July 30, 1968 ff? Vent-0r flana/d L. Perfora United States Patent3,489,161 VARIABLE RESONANT FREQUENCY SPRING- MASS SYSTEM DEVICE DonaldL. Rexford, Schenectady, N.Y., assignor to General Electric Company, acorporation of New York Filed July 30, 1968, Ser. No. 748,753 Int. Cl.F15b /00; F03g 1/00; G05d 16/00 US. Cl. 137-82 24 Claims ABSTRACT OF THEDISCLOSURE A mechanical spring-mass system used as a frequency referencein fluidic systems for controlling speed, temperature and the likeutilizes a fluid pressure signal in the hollow spring element of thespring-mass system for extending the frequency operating range of thesystem without the use of additional moving parts. The spring elementhas a first cross-sectional shape in the absence of the fluid pressuresignal and a second shape in the presence of such signal therebychanging its spring constant and the resonant frequency of thespring-mass system. The fluid signal may be pressurized above or belowambient pressure for deforming the spring element to its second shape.

My invention relates to a mechanical spring-mass system having acontrolled variable resonant frequency, and in particular, to aspring-mass system device wherein the frequency operating range thereofis varied by varying the spring constant as a result of a pressurizedfluid signal applied thereto.

Mechanical-type frequency references are conventionally employed influidic control systems such as for jet engines, and gas and steamturbines for controlling parameters such as speed, temperature and thelike. The mechanical-type frequency references most often employed arethe spring-mass system and the Helmholtz resonator. The spring-masssystem provides a more accurate control over a broad temperature rangethan the Helmholtz resonator type frequency reference but has thedisadvantage of being limited to operation over a small frequency rangeof perhaps :5% of the normal resonant frequency. The Helmholtz resonatorhas the desired wider range of frequency operation but thesignal-to-noise ratio is not sufficiently high for many applications andthe resonator is also relatively temperature sensitive.

Thus, there is need for providing a mechanical type frequency referencewhich is substantially temperature insensitive to thereby provide themore accurate control over a broad temperature range of the spring-masssystem and having the wider frequency operating range of the Helmholtzresonator.

Therefore, one of the principal objects of my invention is to provide animproved mechanical type frequency reference useful in fluidic systemsand having the advantages of both the spring-mass system and Helmholtzresonator type references.

A further object of my invention is to provide an improved mechanicalspring-mass system as a frequency reference having an extended frequencyrange of operation.

Another object of my invention is to obtain the extended frequencyoperating range by means of a fluid pressure signal and without the useof additional moving parts.

In carrying out the objects of my invention, I provide a mechanicalspring-mass system characterized as having a resonant frequencydetermined by the mass and the spring constant of the spring element.The spring element is a hollow member in communication with a fluidpressure signal means such that upon application of a pressure signal,the cross section of the hollow member is deformed and the change insection modulus changes the spring constant thereby varying the resonantfrequency of the spring-mass system. Driving nozzles associated with afirst flapper mounted on the mass body cause mechanical oscillationthereof, and pick-off nozzles associated with a second flapper generatean output signal of the spring-mass system for utilization in anappropriate fluidic circuit.

The features of my invention which I desire to protect herein arepointed out with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings wherein:

FIGURE 1 is a side view of a simplified embodiment of my invention andalso illustrates the changes in cross section of the spring member witha fluid pressure signal applied thereto;

FIGURE 2 is a perspective view, partly in section, of a first embodimentof my invention utilized as a torsional type spring-mass systemj FIGURE3 is a graph illustrating the variation in resonant frequency of thespring-mass system of FIGURE 2 with the pressure of the fluid signalapplied to the spring member;

FIGURE 4 is a perspective view, partly in section, of a secondembodiment of a torsional type spring-mass system device constructed inaccordance with my invention.

FIGURE 5 is a perspective view of a cantilever type spring-mass systemdevice constructed in accordance with my invention.

Referring now in particular to FIGURE 1, there is shown a spring-masssystem of the torsional type wherein a thin-walled hollow spring member10 has both its ends rigidly fixed in position and capable of twistingin torsion as indicated by the arrows in response to mass 11 beingsubjected to a rotational oscillatory motion about its longitudinalaxis. Mass body 11 is rigidly attached to spring member 10 passingtherethrough, and the longitudinal axis of body 11 is aligned with thecenterline axis of the spring member. A plate member 1211 is fixed inposition on mass 11 and is adapted to function as a flapper between twoopposed nozzles 15a which are equally spaced from member 12a in thenonoscillatory state of mass 11. Nozzles 15:: are designated the pickoifnozzles. Mass 11 is caused to oscillate by means of a second flapper 12bfixed in position on mass 11 and interposed between a seiond pair ofopposed, equally spaced, nozzles 15b designated the driving nozzles.Application of a variably pressurized fluid to the driving nozzles,wherein the fluid pressure variation or frequency is in the frequencyoperating range of the spring-mass system, causes rotational oscillationof mass 11 about its longitudinal axis at the corresponding frequencyand resultant oscillatory torsion of spring member 10. The spring-masssystem frequency operating range is defined herein as the frequency bandwithin the resonance curve centered at the natural resonant frequency ofthe springmass system and does not include the frequency range belowsuch band, even though it is recognized that the system is also operabletherein. The amplitude of oscillation is directly proportional to theproximity of the driving nozzle fluid pressure frequency to the resonantfrequency of the spring-mass system. Oscillation of mass 11 generatesunequal pressure (a differentially pressurized signal) within thepick-off nozzles 15a which output of the spring-mass system istransmitted to suitable fluid amplifier circuitry (not shown) forprocessing of such output signal.

A cross section of spring member 10, in the absence of a fluid pressuresignal applied thereto may have the form illustrated in FIGURE 1a orFIGURE lb. In FIG- URE 1a with no signal applied (P= p.s.i.g.) the cr sssection of spring member 10 is an approximate elliptical shape. Uponapplication of a pressurized fluid signal P=P where the pressure isgreater than ambient pressure, the section modulus is changed in thatthe cross section of member 10 approaches a circular shape asillustrated thereby providing a stiffer cross section (increasing thespring constant) and hence increasing the resonant frequency of thespring-mass system.

In like manner,( the no-signal cross section of member 10 may besubstantially circular, but slightly elliptical, as indicated in FIGURE1b and the application of a pressure signal P=P where the pressure isbelow ambient, causes a change in the section modules due to theresulting elliptical shape to thereby provide a less stiff cross section(decreased spring constant) and hence a lower resonant frequency. Theprinciples above described are applicable to each of the embodiments ofmy invention to be hereinafter described.

FIGURE 2 illustrates a first embodiment of my torsional type variablefrequency spring-mass system device. This early embodiment comprises theelements described in FIGURE 1 and further includes a support memberindicated as a whole by numeral 13 for supporting the spring-mass systemand pick-01f and driving means. Support member 13 may be a singleintegral member although in the general case it is a composite structurecomprising side support members 13a, base member 13b and pick-off,driving nozzle support members 130. The foreground member 130 is partlybroken away to more clearly illustrate the driving assembly at thebottom of the mass 11. Side members 13a provide the fixed support forthe two ends of spring member 10 and also provide an inlet for thepressurized fluid signal P supplied to member 10 at one end thereof.Support member 13, or its components 13a, 13b and 130 are eachfabricated from a material such as steel to obtain the necessarystrength and rigidity for adequately supporting the spring-mass system.The ends of spring member 10 are rigidly supported within holes formedin side members 13a in any suitable manner such as by set screws,brazing, or suitable clamping means.

One of such holes 17 indicated at the left end of spring member 10 asviewed by the reader, also forms the inlet passage for pressurized fluidsignal P and is provided with a suitable fitting 14 for externalconnection to the source of signal P (not shown). Pressurized air isgenerally used as the fluid medium for signal P and for supplying thedriving nozzles, however, other gases may also be utilized, as desired.Pressurized liquid may also be employed in some applications.

Mass 11 is a cylindrical body of steel and for the characteristicsillustrated in the graph of FIGURE 3, mass 11 has a length of one inchand diameter of one inch. Spring member 10 passes through mass 11 alongthe longitudinal axis thereof, is rigidly fixed thereto, and isfabricated from beryllium copper tubing having a wall thickness of .005inch, outer diameter of 4 inch and length of approximately 5% inch. Thetubing was deformed into an approximate elliptical cross section havinginch width and A2 inch height outer dimensions. As indicated in thegraph of FIGURE 3. this particular dimensioned springmass system has anatural mechanical resonant frequency of 108 cycles per second in theabsence of any pressure signal P applied to the spring member.

Flappers 12a, 12b are vertically disposed thin flat metal plates rigidlyconnected to mass 11 in a longitudinal direction, and on opposite sidesthereof. First and second pairs of nozzles 15a, 15b are positioned onopposite sides of flappers 12a and 12b to form the pick-off and drivingassemblies, respectively. Each pair of nozzles is aligned and positionedperpendicular to an associated flapper in intercepting relationshiptherewith. Each pair of nozzles is equally spaced from the associatedflapper for the condition of no input to the driving nozzles. Thespacing between the ends of the pick-ofi nozzles 15a and flapper 12a isgenerally in the range of 0.002 to 0.003 inch and the driving nozzle 15bto flapper 12b spacing is generally in the range of 0.010 to 0.015 inchalthough these ranges are not deemed to be a limitation. The nozzles arerigidly supported by members which include holes form ing fluid flowpassages in communication with the nozzles. These holes are providedwith suitable fittings 16b and 16a for respective external connectionsto a first fluidic circuit (not shown) which supplies the pressurizeddriving signals to nozzles 15b and a second fluidic circuit (not shown)which processes the spring-mass system output signal obtained in nozzles15a in a predetermined manner.

As depicted in the graph of FIGURE 3, the resonant frequency versuspressure signal P characteristics of my torsional spring-mass system areonly slightly nonlinear in the range of fluid pressure signals from -10to +30 p.s.i.g. These test results indicate a frequency change ofapproximately /2% per p.s.i. Signal pressure.

A second embodiment of my torsional spring mass system is illustrated inFIGURE 4 wherein base member 13b is partly in section for purposes ofmore clearly illustrating the pick-off, driving assemblies. The chiefdistinction between the embodiments illustrated in FIG- URES 4 and 2 isthe location of the pick-off assembly, being located in a rectangularrecess 21 formed through the bottom surface of base member 13b in theFIGURE 4 embodiment thereby obviating the need for nozzle supportmembers 13c as in the case of the FIGURE 2 embodiment. In FIGURE 2 themass 11 is positioned entirely above base member 13b, whereas in theFIGURE 4 embodiment approximately the lower A of mass 11 is positionedwithin a curved recess 20 formed through the top surface of base member13b and having a shape conforming to the outer cylindrical surface ofmass 11. Recesses 20 and 21 intersect to provide the space necessary forthe oscillatory motion of flappers 12a, 12b. The pair of flappers 12aand 12b are rigidly attached to the bottom-most surface of mass 11 in alongitudinal direction, spaced apart and in alignment with each other.Two pairs of nozzles 15a and 15b are oriented with respect to flappers12a and 12b as described with relation to the FIGURE 2 embodiment toform the pick-off and driving assemblies, respectively. As shown in theillustration, the nozzles are located within the rectangular recess 21,and suitable passages are formed through base member 13b incommunication with the nozzles at one end and terminating in ports (notshown) at the other end for transmitting the fluid pressure signals fromthe pick-off nozzles to suitable fluid amplifier circuitry (not shown),and for supplying the variable pressurized fluid to the driving nozzles.

The FIGURE 2 embodiment illustrates spring member 10 as having theelliptical cross section only in two regions between body 10 and theside support members 1311, whereas in FIGURE 4 the spring member iselliptical along its entire length. Obviously, the latter embodiment ispreferred since a greater length of the spring member may be deformedupon application of pressure signal P, resulting in a greater effectivechange in spring constant.

Although mass 11 may also be a solid body of metal as in the case of theFIGURE 2 embodiment (except for the hollow center provided for springmember 10), my preferred embodiment utilizes a hollow cylindrical body11 which is filled with a heavy type oil for purposes of dampingmechanical oscillations in the spring-mass system. Body 11 also has aninner passage for spring member 10 therethrough. A thin metal plate 22may be interposed in rectangular recess 21 between flappers 12a and 12bfor purposes of eliminating any interaction between the two assembliesof fiappers and nozzles, however, such separating plate is not requiredin most cases since such interaction is usually negligible. In aparticular device constructed in accordance with the embodimentillustrated in FIGURE 4, mass 11 is a hollow, oil-filled cylinder havinga length of /2 inch, outer diameter of 4; inch and wall thickness of0.01 inch. Spring member has an over-all length of 2 /2 inches betweenadjacent side walls of side support members 13a. The spring member 10 isfabricated from beryllium copper tubing having a wall thickness of 0.002inch and an outer width dimension of 4 inch and height of inch in theabsence of pressure signal P. The spring-mass system deviceincorporating these dimensions has a resonant frequency of 400 cyclesper second in the absence of pressure signal P.

A cantilever type embodiment of my variable resonant frequencyspring-mass system device is illustrated in FIG- URE 5 wherein hollowspring member 10 has one end rigidly supported within side supportmember 13a and a second free end which passes into mass 11 and isrigidly attached thereto. Support member 13d,. retains the nozzles in arigidly fixed position. The substantially linear oscillatory motion ofthe free end of the spring-mass system in the FIGURE 5 embodiment is ina vertical direction as indicated by the arrows for the particularorientation of the device having the longer width dimension of springmember 10 in the horizontal plane. This vertical motion is sensed by thepick-otf assembly comprising horizontally disposed flapper 12a rigidlyconnected to mass 11 and a first pair of oppositely disposed, equallyspaced nozzles a oriented perpendicular thereto. The verticaloscillatory motion is induced by the driving assembly comprising asecond horizontally disposed flapper 12b and a second pair of nozzles15:; associated therewith.

In the FIGURE 5 embodiment, mass 11 is illustrated in the form of a cubeor rectangularly shaped body, although it may also be cylindrical. TheFIGURE 5 device may be converted to a horizontal motion device by merelyorienting the device at a 90 angle such that base member 13b isvertically disposed and the longer width dimension of spring member 10is in the vertical plane. The spring member in the FIGURE 5 embodimentis of the same type as in the FIGURES 2. and 4 embodiments, that is,initially formed into a substantially elliptical cross section.

It can be appreciated that the oscillatory motion of mass 11, rotationalin the FIGURES 2 and 4 embodiments and linear in the FIGURE 5embodiment, is very small in amplitude as evidenced by the close spacingof the pick-off nozzles. However, even this amplitude is at least 5 to10 times any steadystate deflection which mass 11 may have when thedevice is operating in its associated fluidic circuit. Also, thedeformation of tube 10 due to signal P is slight, being approximately.030" in the minor axis of an ellipse having minor and major axes of Aand A, respectively, for a signal P=15 p.s.i.g.

The variably pressurized fluid supplied to the driving nozzles in all ofthe hereinabove embodiments may represent a digital or analog signal(i.e., the pressure may vary in square pulse or sine wave form), theonly criterion being that it have the required periodicity or frequencyin the ferquency operating range of the device. Thus, the drivingnozzles may be supplied from a digital or analog type fluidic circuit.The fluid pressure signals generated in the pick-off nozzles, however,are of the sine wave (analog) type and thus an analog type fluidiccircuit would generally be employed to further process such outputsignals.

The fluid signal P supplied to spring member 10 may be a constantpressure on-off type signal or may be programmed to vary in accordancewith a desired variation of a circuit or system parameter. Thus, in thecase of my fluidic frequency reference device being utilized in a speedcontrol circuit, the variable pressurized fluid supplied to the drivingnozzles is a feedback signal having a pressure frequency representingthe actual speed, and the fluid signal P supplied to spring member 10 isa reference signal having a pressure magnitude represent ing thereference or desired speed. Since the reference speed may be varied forparticular purposes, signal P is correspondingly varied in pressuremagnitude to obtain the desired (reference) resonant frequency of thespringmass system which corresponds to the reference speed. Signal P maybe programmed by any conventional means.

It is apparent from the foregoing that my invention attains theobjectives set forth. In particular, my invention provides an improvedmechanical spring-mass system frequency reference useful in fluidicsystems since its various input and output signals are all of the fluidpressure type. My frequency reference has the advantages of both thespring-mass system and Helmholtz resonator type references in that it issubstantially temperature insensitive to there-by provide accuratecontrol over a broad temperature range, and the ability to vary thespring constant obtains an extended frequency range of operation. Thiswider frequency operating range is obtained without the use ofadditional moving parts by means of a fluid Signal applied to the hollowspring member of my spring-mass system. As can be seen from the FIGURE 3graph, the normal i5% frequency range has been increased by at least afactor of two.

Having described three embodiments of my invention, it is believedobvious that modification and variation of my invention is possible inlight of the above teachings. Thus, spring member 10 cross sectionsother than elliptical may be utilized, the criterion being that arelative change in section modulus is obtained upon application ofsignal P to thereby change the spring constant Further, a single drivingnozzle may be utilized instead of the depicted pair of opposed nozzlesin the case wherein the driving pressure signal is of single polarity,without excessive loss in performance of my variable resonant frequencyspring-mass device. Also, a single pick-0E nozzle may be employed.Finally, other types of driving and pick-off assemblies may be utilized,either fluidic or nonfluidic. For example, the driving assembly may beof the electromagnetic type, and the pick-off assembly of the electricalcapacitive type. Thus, it is evident that my invention also has utilityin circuits other than of the fluidic type. It is, therefore, to beunderstood that changes may be made in the particular embodiments of myinvention described which are within the full intended scope of theinvention as defined by the following claims.

What is claimed is: 1. A spring-mass system adapted for use as afrequency reference in fluidic control systems comprising a mechanicalspring-mass system characterized as having a resonant frequencydetermined by the spring constant of the spring elements thereof, andfluid pressure signal means in communication with said spring-masssystem for varying the resonant frequency thereof. 2. A spring-masssystem adapted for use as a frequency reference in fluidic controlcircuits comprisin a mechanical spring-mass system characterized ashaving a resonant frequency determined by the spring constant of thespring element thereof and a limited frequency operating range withinapproximately i5% of the resonant frequency, and

fluid pressure signal means in communication with the spring element forvarying the resonant frequency to thereby extend the frequency operatingrange of the mechanical spring-mass system by varying the springconstant thereof without additional moving parts.

3. The spring-mass system set forth in claim 2 wherein the springelement of said spring-mass system is characterized as having a firstcross sectional shape at a first resonant frequency of said spring-masssystem in the absence of a fluid pressure signal applied thereto fromsaid fluid pressure signal means, and having a second cross sectionalshape at a second resonant frequency in the presence of the fluidpressure signal.

4. A spring-mass frequency reference device comprisa thin-walled hollowspring member having at least one end thereof rigidly fixed in position,

a body having a known mass and a hollow portion forming an entrance forsaid spring member, Said body rigidly attached to said spring member toform a mechanical spring-mass system having a mechanical resonantfrequency determined by the spring constant of said spring member, thespring constant being. a function of the section modulus of said springmember, and

me-ansfor varying the section modulus of said spring member to therebyvary the spring constant and the resonant frequency of said spring-masssystem.

5. The spring-mass frequency reference device set forth in claim 4wherein said section modulus varying means comprises said spring memberenclosed at a first end thereof and open at a second end, the opensecond end adapted to be supplied with a pressurized fluid signal fordeforming said spring member to thereby vary the section modulus thereoffrom its value in the absence of the pressurized fluid signal.

6. The spring-mass frequency reference device set forth in claim 5wherein said spring member comprises a tube having a predeterminedlength and a first cross sectional shape in the absence of thepressurized fluid signal supplied to the interior of said tube, andhaving a second cross sectional shape upon said tube being supplied withthe pressurized fluid signal.

7. The spring-mass frequency reference device set forth in claim 6wherein said thin-walled tube having an approximate elliptical crosssection in the absence of the pressurized fluid signal and asubstantially circular but slightly elliptical cross section upon saidtube being supplied with the signal wherein the signal is at a pressuregreaer than ambient. 8. The spring-mass frequency reference device setforth in claim 6 wherein said thin-walled tube having a substantiallycircular but slightly elliptical cross section in the absence of thepressurized fluid signal and a substantially elliptical cross sectionupon said tube being supplied with the pressurized fluid signal whereinthe signal is at a pressure less than ambient. 9. The spring-massfrequency reference device set forthin claim 6 wherein both ends of saidtube are rigidly fixed in position, the centerline axes of said tube andbody being colinear and said body adapted to be rotationally oscillatedabout its centerline axis thereby causing a corresponding torsionaloscillation of said spring member. 10. The spring-like frequencyreference device set forth in claim 6 wherein only one end of said tubeis rigidly fixed in position and said body is adapted to be oscillatedwith a linear motion about the free end of said tube. v 11. Thespring-mass frequency reference device Set forth in claim 6 and furthercomprising a pair of flapper members rigidly connected to the outersurface of said body, and at least one nozzle associated with eachflapper in intercepting relationship therewith, said nozzle rigidlyfixed in position, a first of said flappers and associated first nozzlescomprising a driving assembly for causing mechanical oscillation of saidbody about its centerline axis upon said first nozzles being suppliedwith a fluid variably pressurized at a periodicity within the frequencyoperating range of said spring-mass system, and

a second of said flappers and associated second nozzles comprising apick-off assembly for generating a fluid pressurized output signalhaving a frequency corresponding to the periodicity of the pressurevariation of the fluid supplied to said first nozzles.

12. The spring-mass frequency reference device set forth in claim 9wherein said body is cylindrical in shape and the rotational oscillationthereof is about its longitudinal axis.

13. A spring-mass frequency reference device comprising a thin-walledtubular member of predetermined length enclosed and rigidly fixed inposition at a first end thereof and open at a second end, said tubularmember having a wall thickness dimension and being fabricated of amaterial such that the cross section of said member is deformable,

a body having a known mass and a hollow interior portion along thelongitudinal axis of said body, said tubular member passing into thehollow portion of said body and rigidly attached therein to form amechanical spring-mass system having a mechanical resonant frequencydetermined by the spring constant of said tubular member,

a pair of flapper members rigidly connected to the outer surface of saidbody longitudinally thereof,

at least one nozzle associated with each flapper in interceptingrelationship therewith, said nozzles rigidly fixed in position,

a first of said flappers and nozzles comprising'a driving assembly forinitiating mechanical oscillation of said body about its longitudinalaxis upon said first nozzle being supplied with a fluid variablypressurized at a periodicity within the frequency operating range ofsaid spring-mass system,

a second of saidflappers and nozzles comprising a pickoff assembly forgenerating a fluid pressurized output signal having'a frequencycorresponding to the periodicity of the pressure variation of the fluidsupplied to said first nozzles, and

means in communication with the open end of said tubular member forvarying the resonant frequency of said spring-mass system to therebyincrease the frequency operating range thereof without any additionalmoving parts.

14. The springmass frequency reference device set forth in claim 13wherein said flappers are positioned on opposite sides of said body.

15. The spring-mass frequency reference device set forth in claim 13wherein said flappers are positioned in alignment with each other.

16. The spring-mass frequency reference device set forth in claim 13wherein said resonant frequency varying means comprises means forsupplying a fluid pressurized signal to the open end of said tubularmember such that in the absence of the signal said tubular member has afirst cross sectional shape and corresponding section modulus and in thepresence of the signal said tubular member has a second cross sectionalshape and section modulus of different magnitude which causes acorresponding variation in the spring constant to thereby vary theresonant frequency as compared to the condition of no signal.

17. The spring-mass frequency reference device set forth in claim 16wherein said tubular member having an approximate elliptical crosssection in the absence of the pressurized fluid signal and asubstantially circular but slightly elliptical cross section upon saidtubular member being supplied with the signal wherein the signal is at apressure greater than ambient, the circular cross section of saidtubular member providing a section modulus of greater magnitude than thesection modulus associated with" the elliptical cross section to therebycause a corresponding increase in the spring constant and therebyincrease the resonant frequency plied with the fluid variablypressurized at periodicity within the frequency operating range of saidspringmass system thereby causing a corresponding torsional oscillationof said tubular member. 21. The spring-mass frequency reference deviceset forth in claim 20 and further comprising a member for, supportingboth ends of said tubular member in rigidly fixed position relative tosaid body, the latter member also supporting the nozzles in rigidlyfixed position relative to their associated flappers.

of the spring-mass system. 22. The spring-mass frequency referencedevice set 18. The spring-mass frequency reference device set forth inclaim 21 wherein forth in claim 16 wherein 1 said supporting memberincludes a recess formed said tubular member having a substantiallycircular but through the bottom surface thereof for containing slightlyelliptical cross section in the absence of the said driving and pick-offassemblies. pressurized fluid signal and a substantially elliptical 23.The spring-mass frequency reference device set cross section upon saidtubular member being supforth in claim 19 and further comprising pliedwith the signal wherein the signal is apressure a member for supportingthe first end of said tubular less than ambient, the substantiallyelliptical cross member in g y Position ffilative to said body, sectionof said tubular member providing a section the latter member alsosupporting the nozzles in mod l of lesser amplitude th th ti m d 20rigidly fixed position relative to their associated flaplus associatedwith the circular cross section to there- P6 7 by cause a correspondingdecrease in the spring con- 24. The sp frequency reference device Setstant and thereby decrease the resonant frequency for h in claim 19wherein of the spring-mass system. the linear oscillatory motion of saidbody about the 19 The Spring mass frequency reference device Set freeend of said tubular member is in a direction fo th in claim 1 wherein 5normal to the greater width dimension of said tubuonly one end of saidtubular member is rigidly fixed in lar member position and said body isoscillated with a linear moi References Clted tion about the free end ofsaid tubular member. UNITED STATES PATENTS 20. The spring-mass frequencyreference device set 3,260 456 7/1966 Boothe 13 X forth in claim 16 wh rin 3,275,015 9/1966 Meier 137-815 both ends of said tubular member arerigidly fixed in position and said body is rotationally oscillated aboutALAN COHAN, Primary Examiner its longitudinal axis upon said firstnozzles being sup- US. Cl. X.R.

